A procedure has been developed in our laboratory for the isolation of rat kidney transamidinase to what appears to be a homogenous state. Two forms of the enzyme have been found. The enzyme isolated from rats fed a complete diet has 2 times the specific activity as that isolated from rats fed the same diet supplemented with creatine. So far, we have been unable to detect any reason for the differences in specific activities. One step in our purification procedure is the use of hydroxylapatite. This step has been found not to be reproducible. We are at the present time trying to develop a procedure for the isolation of the enzyme that does not employ the use of hydroxylapatite. When a new isolation procedure has been developed, attempts will be continued to find the reason for the differences in specific activities of the 2 transamidinases.